This invention is related to highly localized Infrared (IR) spectra on a sample surface utilizing an Atomic Force Microscope (AFM) and a variable wavelength pulsed IR source.
IR spectroscopy is a useful tool in many analytical fields, such as polymer science and biology. It would be particularly useful to perform IR spectroscopy on a highly localized scale, on the order of biological organelles or smaller, at various points on a sample surface. Such a capability would provide information about the composition of surfaces, such as location of different materials or molecular structures. Recently, one of the inventors has developed a technique based on use of an AFM in a unique fashion to produce such localized spectra. This work was described in a publication entitled “Local Infrared Microspectroscopy with Sub-wavelength Spatial Resolution with an Atomic Force Microscope Tip Used as a Photo-thermal Sensor” Optics Letters, Vol. 30, No. 18, Sep. 5, 2005. Those skilled in the art will comprehend the details of the technique in the public.
AFMs are known in the art. The important aspects of an AFM for the current invention are shown in FIG. 1. A light beam 1 is directed at an angle to a cantilever arm 2 which reflects the beam to a photo-detector 4. Typically the photodetector is a 4-quadrant type, and when the cantilever is placed in the AFM the lever is positioned such that the reflected beam is centered on the photo-detector. The cantilever typically has a probe tip in contact with sample 3. Deflection of the cantilever vertically due to contact with the sample causes the beam spot to move on the detector, generating a difference signal from the detector quadrants. This type of AFM setup is called an optical lever arm, and commercially available AFMs using this technique can measure deflections of the lever on a sub-angstrom scale. There are other options to detect the deflection of the cantilever which can also be used, such as optical interferometry. Other deflection detectors may not generate as much information from the sample as the beam deflection detection. Not shown, but typically present in an AFM, is a scanner to move the sample laterally relative to the tip, and feedback electronics which typically servo the sample or tip up and down in response to height variations of the sample to keep the spot centered on the detector. This vertical servo signal vs lateral position creates a topographical map of the surface which in, commercial AFMs, approaches atomic resolution. A wide variety of variations of the AFM exist with different types of probes and so on for measurements other than topography. For instance, in a co-pending application by some of the inventors of this application, a version of an AFM configured to measure thermal properties of a surface is described.
The AFM set-up used for the published work on IR spectroscopy is shown schematically in FIG. 2. In this set-up, the sample 3 is mounted on a ZnSe prism 8, or prism made from other suitable materials, which does not absorb the radiation of interest. A pulsed IR source 9, in this case a Free Electron Laser beam, is directed into the prism. The prism is made at an angle such that the beam is in Total Internal Reflection in order for the beam to be propagative in the sample and evanescent in the air. Thus only the sample is significantly exposed to the laser radiation, and the AFM probe 2 is minimally exposed to beam 9. The Free Electron Laser (FEL) is an IR source that is both variable in wavelength and has a pulsed output. The probe 2 is placed at a point on the sample by the scanner 6 and is held at an average height by feedback electronics 5. Both the vertical and lateral deflection signal directly from the photo-detector 4, as well as the feedback signal, are monitored at 7.
Referring to FIG. 3, when the FEL is pulsed, the sample 3 may absorb some of the energy, resulting in a fast thermal expansion of the sample as shown in the Figure. This has the effect of a quick shock to the cantilever arm 2, which if the ability of the cantilever to respond to this shock is slower than the shock will result in exciting a resonant oscillation in the cantilever arm. The resonant oscillation decays as shown in the Figure. Because the FEL energy is ideally contained within the sample, this shock is due primarily to rapid sample expansion as minimal IR energy is absorbed by the cantilever itself. Although the probe is kept in contact with the surface by the feedback electronics, the resonant signal is too fast for the feedback electronics, but can be observed directly from the photodetector. Thus the cantilever rings in the manner shown in FIG. 3 while still in contact with the surface, an effect called “contact resonance”. The absolute deflection, amplitude and frequency characteristics of the contact resonance vary with the amount of absorption as well as other properties, such as the local hardness, of the localized area around the probe tip. Also, depending on the direction of the expansion, vertical resonances, lateral resonances or both can be excited. If the tip is to the side of the absorbent material, this will typically cause a stronger lateral response in the cantilever. By repeating the above process at varying wavelengths of the FEL, an absorption spectra on a very localized scale is achieved. By scanning the probe to various points on the sample surface and repeating the spectra measurement, a map of IR spectral surface characteristics can be made. Alternatively, the wavelength of the FEL can be fixed at a wavelength that is characteristic of absorption of one of the components of the sample. The probe can then be scanned across the sample surface and a map of the location of that component can be generated.
Although the set-up as described produced positive results, there is no real possibility of commercializing the set-up as published. For one thing, the requirement to place the sample on a prism to prevent exposing the lever to the IR pulse would limit the possible applications. The actual signals generated can be small thus requiring averaging of the signal and limiting the bandwidth of the technique. More sensitivity would be required to address a wider range of potential samples. Moreover, an FEL is a very expensive and large device and only a few exist in the world. Also, up to this point, the technique is not quantitative in terms of the amount of absorption that occurs in the local area. Determining the absorption from the contact resonance amplitude as well as the measurement of some thermal properties of the sample would improve the ability of the technique to identify materials. Thus a variety of issues must be addressed in order to take the published technique from a laboratory set-up to a commercial analytical instrument. The present invention addresses the commercialization issues.